1. Field of the Invention
The present invention relates to a positive resist composition used in a manufacturing process of semiconductor devices, such as IC, manufacture of circuit substrates for liquid crystals, thermal heads and the like, and lithographic processes of other photo-fabrication, and also the invention relates to a pattern-forming process using the same. In particular, the invention relates to a positive resist composition suitable for exposure by an immersion projection exposure apparatus with far ultraviolet rays of the wavelength of 300 nm or less as the light source, and a pattern-forming process using the positive resist composition.
2. Description of the Related Art
With the progress of fining of semiconductor elements, shortening of the wavelengths of exposure light source and increasing of the numerical aperture (high NA) of projection lens have advanced, and now exposure apparatus of NA 0.84 using an ArF excimer laser having the wavelength of 193 nm as the light source have been developed, which can be expressed by the following equations as generally known:(Resolution)=k1·(λ/NA)(Depth of focus)=±k2·λ/NA2 wherein λ is the wavelength of exposure light source, NA is the numerical aperture of the projection lens, and k1 and k2 are the coefficients concerning the process.
For further higher resolution by the shortening of wavelengths, an exposure apparatus with an F2 excimer laser having the wavelength of 157 nm as the light source has been studied, but the materials of lens for use in the exposure apparatus for the shortening of wavelengths and the materials of photoresist are extremely restricted, so that the realization of the reasonable manufacturing costs of the apparatus and materials and quality stabilization are very difficult, as a result, there are possibilities of missing an exposure apparatus and a photoresist having sufficient performances and stabilities within a required period of time.
As a technique for increasing resolution in optical microscopes, a so-called immersion method of filling a liquid of high refractive index (hereinafter sometimes referred to as “immersion liquid”) between a projection lens and a sample has been conventionally known.
As “the effect of immersion”, the above resolution and depth of focus can be expressed by the following equations in the case of immersion, with λ0 as the wavelength of the exposure light in the air, n as the refractive index of immersion liquid to the air, and NA0=sin θ with θ as convergence half angle of the ray of light:(Resolution)=k1·(λ0/n)/NA0 (Depth of focus)=±k2·(λ0/n)/NA02 
That is, the effect of immersion is equivalent to the case of using exposure wavelength of the wavelength of 1/n. In other words, in the case of the projection optical system of the same NA, the depth of focus can be made n magnifications by immersion. This is effective for every pattern form, and further, this can be combined with super resolution techniques such as a phase shift method and a deformation lighting method now under discussion.
As the example of the application of this effect to the formation of micro-fine image pattern of semiconductor element, JP-B-63-49893 (the term “JP-B” as used herein refers to an “examined Japanese patent publication”) and JP-A-6-124873 (the term “JP-A” as used herein refers to an “unexamined published Japanese patent application”) are known, but resists suitable for immersion exposure techniques are not disclosed in these patents.
It is appointed in JP-A-10-303114 that the control of the refractive index of an immersion liquid is important, since the variation of the refractive index of an immersion liquid causes the deterioration of a projected image due to the wave surface aberration of exposure apparatus, and controlling the temperature coefficient of the refractive index of an immersion liquid to a certain range, and water added with additives for reducing surface tension or increasing the degree of surface activity are disclosed as a preferred immersion liquid. However, the specific additives are not disclosed and resists suitable for the technique of immersion exposure are not also discussed.
From the advent of the resist for a KrF excimer laser (248 nm) on, an image-forming method that is called chemical amplification is used as the image-forming method of the resist for compensating for the reduction of sensitivity by light absorption. To explain the image-forming method of positive chemical amplification by example, this is an image-forming method of exposing a resist to decompose an acid generator in the exposed area to thereby generate an acid, utilizing the generated acid as the reactive catalyst to change an alkali-insoluble group to an alkali-soluble group by the bake after exposure (PEB: Post Exposure Bake), and removing the exposed area by alkali development.
The resist for ArF excimer laser (193 nm) using the mechanism of chemical amplification is now becoming the main stream, but the resist is not sufficient in performance for development defects occurring after development, and further improvement is required.
In addition, when chemical amplification resist is applied to the immersion exposure technique for further fine pattern forming, the acid on the surface of the resist generated at the time of exposure migrates to the immersion liquid, and the acid concentration on the exposed surface changes. This phenomenon is thought very similar to the acid deactivation of the surface of the exposed area caused by basic contamination of a trace amount of several ppb level from the environment at the time of post exposure time delay (PED) between exposure and PEB, which became an issue at the beginning of the development of a chemical amplification positive resist, but the influence of immersion exposure on a resist and its mechanism are still unclear. When a chemical amplification resist that causes no lithographic problem in general dry exposure is subjected to immersion exposure, there are problems that depth of focus (hereinafter referred to as “DOF”) as the degree of allowance to focus variation and profile deteriorate.
Further, in performing exposure with a scanning system immersion exposing apparatus as described in OPTRONICS No. 4 2003 in immersion exposure process, exposure speed lowers if the immersion liquid does not transfer following the transfer of the lens, so that there is the fear that the productivity is influenced. When the immersion liquid is water, a resist film is preferably hydrophobic for the follow-up ability of water, on the other hand when a resist film is hydrophobitized, image performance of the resist is adversely influenced such as by the increase of the occurrence of scum, and so the improvement is required.